I would ALWAYS suspect poor connections in a situation like this. Clean battery terminals AND inside the clamps.......
Remove and clean contacts on starter and relay. Dremel tool wire brush works WAY good for this.
Also clean ANY ground connections to frame and engine block.
"For Gawd's sake, put a RELAY in there or you could burn the whole damn Jeep to the ground" ...this quote brought to you by people who do not understand the law.....Kirchoff's, Ohm's, or Faraday's.
Violation of KISS shall always ring false in TRUE engineering. - ME___
Had 75CJ5, 89YJ, 93YJ. Have 82CJ7 & 95YJ____Not one burnt yet due to lack of relays.